Add LICENSE.txt to pypi tarball

LICENSE.txt is not available in pypi tarball (https://files.pythonhosted.org/packages/7f/bd/aa36c1a1d876757e3fa365e6c455097ebd3f2e8e7ded23e75901ff9c9ecf/yatl-20200430.1.tar.gz), would it possible to add it in the next releases?